kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 133 insertions, 0 deletions

diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
new file mode 100644
index 000000000..b99122cb2
--- /dev/null
+++ b/samples/livepatch/livepatch-shadow-fix2.c
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
+ */
+
+/*
+ * livepatch-shadow-fix2.c - Shadow variables, livepatch demo
+ *
+ * Purpose
+ * -------
+ *
+ * Adds functionality to livepatch-shadow-mod's in-flight data
+ * structures through a shadow variable.  The livepatch patches a
+ * routine that periodically inspects data structures, incrementing a
+ * per-data-structure counter, creating the counter if needed.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * This module is not intended to be standalone.  See the "Usage"
+ * section of livepatch-shadow-mod.c.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+#include <linux/slab.h>
+
+/* Shadow variable enums */
+#define SV_LEAK		1
+#define SV_COUNTER	2
+
+struct dummy {
+	struct list_head list;
+	unsigned long jiffies_expire;
+};
+
+static bool livepatch_fix2_dummy_check(struct dummy *d, unsigned long jiffies)
+{
+	int *shadow_count;
+
+	/*
+	 * Patch: handle in-flight dummy structures, if they do not
+	 * already have a SV_COUNTER shadow variable, then attach a
+	 * new one.
+	 */
+	shadow_count = klp_shadow_get_or_alloc(d, SV_COUNTER,
+				sizeof(*shadow_count), GFP_NOWAIT,
+				NULL, NULL);
+	if (shadow_count)
+		*shadow_count += 1;
+
+	return time_after(jiffies, d->jiffies_expire);
+}
+
+static void livepatch_fix2_dummy_leak_dtor(void *obj, void *shadow_data)
+{
+	void *d = obj;
+	int **shadow_leak = shadow_data;
+
+	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
+			 __func__, d, *shadow_leak);
+	kfree(*shadow_leak);
+}
+
+static void livepatch_fix2_dummy_free(struct dummy *d)
+{
+	int **shadow_leak;
+	int *shadow_count;
+
+	/* Patch: copy the memory leak patch from the fix1 module. */
+	shadow_leak = klp_shadow_get(d, SV_LEAK);
+	if (shadow_leak)
+		klp_shadow_free(d, SV_LEAK, livepatch_fix2_dummy_leak_dtor);
+	else
+		pr_info("%s: dummy @ %p leaked!\n", __func__, d);
+
+	/*
+	 * Patch: fetch the SV_COUNTER shadow variable and display
+	 * the final count.  Detach the shadow variable.
+	 */
+	shadow_count = klp_shadow_get(d, SV_COUNTER);
+	if (shadow_count) {
+		pr_info("%s: dummy @ %p, check counter = %d\n",
+			__func__, d, *shadow_count);
+		klp_shadow_free(d, SV_COUNTER, NULL);
+	}
+
+	kfree(d);
+}
+
+static struct klp_func funcs[] = {
+	{
+		.old_name = "dummy_check",
+		.new_func = livepatch_fix2_dummy_check,
+	},
+	{
+		.old_name = "dummy_free",
+		.new_func = livepatch_fix2_dummy_free,
+	}, { }
+};
+
+static struct klp_object objs[] = {
+	{
+		.name = "livepatch_shadow_mod",
+		.funcs = funcs,
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_shadow_fix2_init(void)
+{
+	return klp_enable_patch(&patch);
+}
+
+static void livepatch_shadow_fix2_exit(void)
+{
+	/* Cleanup any existing SV_COUNTER shadow variables */
+	klp_shadow_free_all(SV_COUNTER, NULL);
+}
+
+module_init(livepatch_shadow_fix2_init);
+module_exit(livepatch_shadow_fix2_exit);
+MODULE_DESCRIPTION("Live patching demo for shadow variables");
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");